Numerous products need to be adapted to the individual shape of the object for which they are intended, such as e.g. a body part. For instance, many products that are in contact with the human body, such as footwear, shirts, pants, gloves, orthopedic articles such as elastic stockings and bandages, sport supports, spectacle frames, etc., need to fit well, i.e. their three-dimensional geometry must correspond to the three-dimensional shape in space of the respective part of the body. These fitting requirements do not only apply to products which need to be adapted to the human body but, in a broader sense, also to products or components which need to be adapted to technical bodies. To mention an example, a plastic casing is to be adapted to the mechanical base frame of a household appliance.
The sensing of the 3D shape, i.e. the three-dimensional geometry, of bodies is nowadays done by means of so-called 3D scanners, for example, which usually operate using methods based on stripe projection, laser triangulation, stereo measurement, photogrammetry, and the methods of point-by-point scanning of the distance of the object of the body to be sensed using a distance measuring device. Where these devices serve to measure the human body, they are often referred to as “body scanners”, while they are called “3D digitizers” for more technical applications. In all of these methods, relatively complex and, hence, expensive electrooptical systems are employed. For instance, currently used body scanners cost between £100,000 and £250,000.
In European Patent EP 0 760 622, “Digitized Sensing Process and Arrangement for the Three-Dimensional Shape in Space of Bodies or Body Parts”, the inventor, R. Massen, describes an extremely low-cost way of sensing the 3D shape of body parts which is based on multiple camera or single camera photogrammetry. In this process, the body part to be digitized is covered with a tight-fitting elastic envelope which is provided with target markers adapted to be evaluated photogrammetrically. The body part is imaged from different spatial positions which need not necessarily be known or fixed in space, and the space coordinates of the target markers are determined from the overlapping individual images with the aid of photogrammetric methods of reconstruction, which allows to determine the three-dimensional shape of the body.
In German Patent Application 100 25 922.7, the inventor, R. Massen, describes various methods of how an automatic registration, i.e. an association of the punctiform target markers in image pairs, can be carried out more easily by way of encoding the background between punctiform target markers in the form of area markers. For instance, it is described how corresponding regions in two images can be determined in a simple manner using the methods of digital color image processing, by a colored marking over an area of background regions of the punctiform target markers based on a simple color classification.
For a photogrammetric reconstruction of an object to be possible, it is necessary that pairs of image recordings overlap to a sufficient extent, i.e. that as many identical target markers as possible are contained in two corresponding image recordings which have an at least partially overlapping image area.
The 3D coordinates of the target markers can be calculated the more easily and accurately the better the camera positions that were assumed by the camera while recordings of a pair of images with overlapping image areas were obtained are aligned in relation to each other. Since, as a rule, the inner parameters of the camera such as the focal length, the number and size of the pixels (in the case of a digital camera), the position of the optical axis in relation to the focal plane, etc., were determined by a previous calibration and need to be constant throughout the recordings, the focal length needs to be kept constant. This means that no zoom functions may be used and that autofocus means must also be switched off. Accordingly, sharply focused images can only be attained if the distance of the camera to the body part to be scanned is approximately constant in all positions in space.
This condition is only very difficult to fulfill for an operator who leads the camera free-handed around the body part to be sensed while obtaining the image recordings to be evaluated photogrammetrically, which makes it considerably more difficult, especially for a layman, to obtain the image recordings in a simple manner.
The background markings that combine a plurality of punctiform target markers to form an area as described in German Patent Application 100 25 922.7 are already very useful to the automatic photogrammetric evaluation of the image recordings of the body part marked by e.g. a tight-fitting envelope. However, the demands made on the image processing methods are still high if the corresponding target markers from image pairs have to be found, where nothing is known about the orientation of the camera in space while the recordings are obtained. Search procedures are then required which are involved and, hence, prone to error, in order to prevent that target markers in the respectively analyzed pair of image recordings are associated with each other which in fact do not correspond.